1. Field of the Invention
The present invention relates to a starting device for motor vehicles that can provide a creep running function and a torque increase function without a torque converter.
2. Description of the Related Art
A starting device for motor vehicles of this kind is disclosed in Japanese patent laying-open publication No. 2003-35353. This starting device is composed of an auxiliary transmission arranged between an engine and a main transmission with a forward-backward shifting mechanism. The auxiliary transmission is equipped with a planetary gear set, an one-way clutch for controlling the gear set, and a hydraulically operated clutch for controlling the gear set.
The planetary gear set selectively provides a direct drive ratio and a reduction gear ratio larger than 1.00. The direct drive ratio is obtained by engaging the clutch and disengaging the one-way clutch, while the reduction gear ratio is obtained by engaging the one-way clutch and disengaging the clutch. The clutch is also capable of producing creep torque by slipping the clutch. The clutch has drive friction plates and driven friction plates and acts as a friction element. The one-way clutch acts as a brake when it is engaged. Therefore, this conventional starting device can provide a creep running function and a torque increase function, which provides functions similar to a torque converter. This enables to eliminate a torque converter, thereby bringing improvement in fuel economy and cost saving.
The above known conventional starting device, however, encounters a problem that a friction element, such as the clutch or a brake, generates heat due to slip for a smooth vehicle start and/or creep of a motor vehicle, and its caloric value sometimes exceeds a permissible value when the creeping time becomes long, causing premature deterioration and/or burnout of the friction element.
Particularly, in the conventional starting device, operation frequency is different between a forward friction element (the clutch) slipped when the vehicle creeps forward and a reverse friction element (the brake) slipped when the vehicle creeps backward, and load due to slip of the friction element in vehicle creeping is not detected for protecting it. This can not avoid the premature deterioration and/or the burnout of the friction element due to excessive load.
It is, therefore, an object of the present invention to provide a starting device for motor vehicles which overcomes the foregoing drawbacks and can produce creep torque by a slip of a friction element with suppressing premature deterioration or burnout of the friction element.